Lash adjuster

ABSTRACT

A lash adjuster is provided of which the return spring is never compressed excessively when the adjusting screw is pushed into the nut member, and of which the external thread on the outer periphery of the adjusting screw can be formed easily by rolling. 
     The lash adjuster  1  includes a nut member  12  having an internal thread  13 , an adjusting screw  15  having an external thread  14  on its outer periphery which is in threaded engagement with the internal thread  13 , and a return spring  17  biasing the adjusting screw  15  in the direction to protrude from the nut member  12 . The adjusting screw  15  pivotally supports an arm  7  of a valve gear at its end protruding from the nut member  12 . The adjusting screw  15  has at its portion protruding from the nut member  12  a stopper portion  23  having a larger diameter than the inner diameter of the nut member  12 . By supporting the stopper portion  23  on the open end of the nut member  12 , it is possible to restrict the movement of the adjusting screw  15  when the adjusting screw  15  is pushed into the nut member  12.

TECHNICAL FIELD

This invention relates to a lash adjuster mounted in an engine valvegear.

BACKGROUND ART

Known valve gears for moving a valve provided at an intake port or anexhaust port of an engine include one comprising an arm pivotable aboutone end thereof and adapted to be pushed down at its central portion,thereby pushing down a valve stem at the other end (swing arm type valvegear), one comprising an arm pivotable about its central portion andadapted to be pushed up by a cam at one end thereof, thereby pushingdown a valve stem at the other end (rocker arm type valve gear), and onecomprising a valve lifter vertically movably supported and adapted to bepushed down by a cam, thereby pushing down a valve stem (direct typevalve gear).

In these valve gears, gaps between their component parts may change dueto differences in thermal expansion between component parts, which maycause noise and compression leakage. Also, when the sliding parts of thevalve gear become worn too, gaps between component parts of the valvegear may change, which may also cause noise.

In order to prevent such noise and compression leakage, ordinary valveadjusters include a lash adjuster for absorbing gaps between componentparts of the valve gear.

One known lash adjuster used in a swing arm type valve gear comprises anut member inserted in a mounting hole formed in the top surface of acylinder head, an adjusting screw having an external thread on its outerperiphery which is in threaded engagement with an internal thread formedon the inner periphery of the nut member, and a return spring biasingthe adjusting screw in the direction to protrude upwardly from the nutmember, the adjusting screw pivotally supporting the arm of the valvegear with its end protruding from the nut member (Patent documents 1 and4).

One know lash adjuster used in a direct type valve gear comprises alifter body vertically slidably inserted in a guide hole formed in acylinder head, a nut member fixed to the lifter body so as to bevertically movable together with the lifter body, an adjusting screwhaving an external thread on its outer periphery which is in threadedengagement with an internal thread formed on the inner periphery of thenut member, and a return spring biasing the adjusting screw in thedirection to protrude downwardly from the nut member, the adjustingscrew pressing the valve stem of the valve gear with its end protrudingfrom the nut member (Patent document 2).

One known lash adjuster used in a rocker arm type valve gear comprises anut member inserted in a mounting hole formed in the bottom surface ofthe arm which pivots as the cam rotates, an adjusting screw having anexternal thread on its outer periphery which is in threaded engagementwith an internal thread formed on the inner periphery of the nut member,and a return spring biasing the adjusting screw in the direction toprotrude downwardly from the nut member, the adjusting screw pressingthe valve stem of the valve gear with its end protruding from the nutmember (Patent documents 3 and 4).

In Patent documents 1-3, the return spring is a coil spring that appliesan axial force to the adjusting screw that tends to move the adjustingscrew in the direction to protrude from the nut member. In Patentdocument 4, the return spring is a torsion spring that applies torque tothe adjusting spring that tends to move the adjusting screw in thedirection to protrude from the nut member.

In these lash adjusters, as a cam rotates and a load that tends to pushthe adjusting screw into the nut member is applied, the pressure flankof the external thread of the adjusting screw is supported on thepressure flank of the internal thread of the nut member, so that theadjusting screw is axially fixed in position.

If the relative position between the arm and the valve stem changes duee.g. to thermal expansion of the valve gear, according to the degree ofchange in the relative position, the adjusting screw axially moves inthe nut member while rotating, thereby absorbing gaps between componentparts of the valve gear.

Patent document 1: JP Patent Publication 2005-273510APatent document 2: JP Patent Publication 2003-227318APatent document 3: JP Patent Publication 2006-132426APatent document 4: JP Utility Model Publication 64-34407

DISCLOSURE OF THE INVENTION Object of the Invention

With these types of lash adjusters, in an ordinary situation, onlyslight slip occurs between the external thread of the adjusting screwand the internal thread of the nut member. But if abnormal loads orvibrations act on the adjusting screw due to over-speed of the engine,if the frictional resistance between the pressure flanks decreases dueto wear of the external thread and the internal thread, or if theadjusting screw is suddenly and rapidly pushed in due to sudden thermalexpansion of the valve gear, unnecessarily large slip may occur betweenthe external thread of the adjusting screw and the internal thread ofthe nut member.

As a result, the adjusting screw is markedly pushed into the nut member,which may cause breakage of the return spring due to over-compression orseparation of the arm.

One way to prevent the return spring from being excessively compressedand broken when the adjusting screw is pushed into the nut member wouldbe to provide a tubular extension at the end of the adjusting screwlocated in the nut member so as to surround the return spring. Theextension restricts the movement of the adjusting screw by abutting thebottom of the nut member. But with this arrangement, since the tubularextension of the adjusting screw is low in rigidity, when the externalscrew is formed on the outer periphery of the adjusting screw byrolling, the external screw tends to be formed incompletely.

An object of the present invention is to provide a lash adjuster ofwhich the return spring is never compressed excessively when theadjusting screw is pushed into the nut member, and of which the externalthread on the outer periphery of the adjusting screw can be formedeasily by rolling.

Means to Achieve the Object

In order to achieve this object, the adjusting screw is provided with astopper portion at its portion protruding from the nut member, thestopper portion having a diameter larger than the inner diameter of thenut member. The stopper portion abuts an open end of the nut member,thereby restricting movement of the adjusting screw when the adjustingscrew is pushed into the nut member.

The stopper portion may have a tapered surface on its outer peripherywhich is configured to abut a tapered surface formed on the open end ofthe nut member. With this arrangement, when the adjusting screw ispushed into the nut member, the tapered surfaces contact each other,thereby preventing inclination of the adjusting screw. This helps theadjusting screw to protrude smoothly thereafter.

Alternatively, the stopper portion may have a flat surface on its outerperiphery which extends perpendicular to the axial direction andconfigured to abut a flat surface formed on the open end of the nutmember so as to extend perpendicular to the axial direction. With thisarrangement too, when the adjusting screw is pushed into the nut member,the flat surfaces contact each other, thereby preventing inclination ofthe adjusting screw, which helps the adjusting screw to protrudesmoothly thereafter.

The stopper portion may be integrally provided at the portion of theadjusting screw protruding from the nut member, or may be a separatemember from the adjusting screw. In the latter case, such a separatestopper member may be a snap ring fitted on the outer periphery of theportion of the adjusting screw protruding from the nut member.

This invention is applicable to a lash adjuster wherein the returnspring is a compression spring that applies an axial force to theadjusting screw that tends to push the adjusting screw out of the nutmember. In this case, the external thread and the internal thread mayhave a serration-shaped section with their pressure flanks having alarger flank angle than their clearance flanks.

This invention is also applicable to lash adjuster wherein the returnspring is a torsion spring that applies torque to the adjusting screwthat tends to push the adjusting screw out of the nut member. In thiscase, the external thread and the internal thread may beserration-shaped threads, trapezoidal threads or triangular threads. Thetorsion spring may be one of a torsion coil spring, a spiral spring anda volute spring.

This invention is applicable e.g. to the following lash adjusters.

1) A lash adjuster wherein the nut member is inserted in a receivinghole formed in a top surface of a cylinder head, and wherein theadjusting screw pivotally supports an arm of a valve gear at its endprotruding from the nut member.2) A lash adjuster of any wherein the nut member is fixed to a lifterbody vertically slidably inserted in a guide hole formed in a cylinderhead, and wherein the adjusting screw presses a valve stem of a valvegear at its end protruding from the nut member.3) A lash adjuster wherein the nut member is inserted in a receivinghole formed in a bottom surface of an arm that pivots as a cam rotates,and wherein the adjusting screw presses a valve stem of a valve gear atits end protruding from the nut member.

The adjusting screw may comprises a pivot member axially slidablyinserted in the nut member, an externally threaded member supporting anend of the pivot member located in the nut member and having theexternal thread on its outer periphery, and an elastic member disposedbetween the pivot member and the externally threaded member.

In this case, the pivot member preferably has a square hole formed inits end located in the nut member, and the externally threaded memberpreferably has a square shaft engaged in the square hole, whereby thepivot member and the externally threaded member are rotatable togetherdue to the engagement of the square shaft in the square hole. With thisarrangement, since the externally threaded member can be rotated byrotating the pivot member, the externally threaded member can be easilymounted in the nut member.

ADVANTAGES OF THE INVENTION

With the lash adjuster according to this invention, when the adjustingscrew is pushed into the nut member, the stopper portion abuts the openend of the nut member, thereby restricting movement of the adjustingscrew. This prevents over-compression of the return spring.

With this lash adjuster, since the stopper member for restricting themovement of the adjusting screw is provided at the portion of theadjusting screw protruding from the nut member, it is not necessary toprovide a tubular extension for restricting the movement of theadjusting screw by abutting the bottom member at the end of theadjusting screw located in the nut member. Thus, the external thread onthe outer periphery of the adjusting screw can be formed easily byrolling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a valve gear including a lash adjusteraccording to a first embodiment of the present invention.

FIG. 2 is an enlarged sectional view of the lash adjuster shown in FIG.1, showing the state in which its adjusting screw is pushed in.

FIG. 3 is an enlarged sectional view of a modification of the lashadjuster shown in FIG. 2.

FIG. 4 is an enlarged sectional view of a further modification of thelash adjuster of FIG. 2.

FIG. 5 is an enlarged sectional view of a still further modification ofthe lash adjuster of FIG. 2.

FIG. 6 is a front view of a valve gear including a lash adjusteraccording to a second embodiment of the present invention.

FIG. 7 is a front view of a valve gear including a lash adjusteraccording to a third embodiment of the present invention.

FIG. 8 is an enlarged view of a modification of the return spring shownin FIG. 2.

FIG. 9 is an enlarged view of a modification of the return spring shownin FIG. 6.

FIG. 10 is an enlarged view of a modification of the return spring shownin FIG. 7.

FIG. 11 is an enlarged sectional view of a lash adjuster according to afourth embodiment of the present invention.

FIG. 12 is an enlarged sectional view of a modification of the returnspring shown in FIG. 11.

FIG. 13 is an enlarged sectional view of a further modification of thereturn spring shown in FIG. 11.

FIG. 14 is an enlarged sectional view of a still further modification ofthe return spring shown in FIG. 11.

FIG. 15 is a sectional view taken along line XV-XV of FIG. 14.

DESCRIPTION OF THE NUMERALS

-   1. Lash adjuster-   2. Cylinder head-   7. Arm-   11. Receiving hole-   12. Nut member-   13. Internal thread-   14. External thread-   15. Adjusting screw-   15A. Pivot member-   15B. Externally threaded member-   15C. Spring washer-   17. Return spring-   18. Pressure flank-   19. Clearance flank-   23. Stopper portion-   24, 25. Tapered surface-   26, 27. Flat surface-   28. Snap ring-   31. Lash adjuster-   32. Cylinder head-   35. Valve stem-   39. Guide hole-   40. Lifter body-   41. Nut member-   42. Internal thread-   43. External thread-   44. Adjusting screw-   45. Return spring-   51. Stopper portion-   61. Lash adjuster-   65. Valve stem-   67. Arm-   71. Cam-   73. Nut member-   74. Adjusting screw-   75. Return spring-   76. Receiving hole-   77. Internal thread-   78. External thread-   82. Stopper portion-   91. Lash adjuster-   94, 95. Flat surface-   96. Square hole-   97. Square shaft

BEST MODE FOR EMBODYING THE INVENTION

FIG. 1 shows a valve gear including the lash adjuster 1 according to thefirst embodiment of the present invention. This valve gear includes avalve 4 for an intake port 3 formed in a cylinder head 2 of an engine, avalve stem 5 connected to the valve 4, and an arm 7 that pivots as a cam6 rotates.

The valve stem 5 extends upwardly from the valve 4 and is slidablyinserted through the cylinder head 2. An annular spring retainer 8 isfixed to the outer periphery of the valve stem 5 at its upper portion. Avalve spring 9 is mounted between the bottom surface of the springretainer 8 and the top surface of the cylinder head 2. The valve spring9 biases the valve stem 5 upwardly through the spring retainer 8,thereby seating the valve 4 on a valve seat 10.

As shown in FIG. 2, the lash adjuster 1 comprises a tubular nut member12 received in a receiving hole 11 formed in the top surface of thecylinder head 2, an adjusting screw 15 having an external thread 14 onits outer periphery at its lower portion which is in threaded engagementwith an internal thread 13 formed on the inner periphery of the nutmember 12, a bottom member 16 fixed to the bottom end of the nut member12, and a return spring 17 mounted between the adjusting screw 15 andthe bottom member 16.

The external thread 14 and the internal thread 13 each have a pressureflank 18 for receiving an axial load that tends to push the adjustingscrew 15 into the nut member 12, and a clearance flank 19, and have aserration-shaped section with the pressure flank 18 having a largerflank angle than the clearance flank 19.

The return spring 17 is a compression coil spring having its bottom endsupported on the bottom member 16 and applying at its top end an axialforce to the adjusting screw 15 through a spring seat 20 that tends toprotrude the adjusting screw 15 upwardly from the nut member 12. The endportion of the adjusting screw 15 that is inserted in the nut member 12has a tubular shape, and receives the spring seat 20.

The end of adjusting screw 15 protruding from the nut member 12 isengaged in a recess 21 formed in the bottom surface of the arm 7 at oneend thereof. The arm 7 is thus pivotally supported about the protrudingend of the adjusting screw 15. The bottom surface of the arm 7 at theother end is in contact with the top end of the valve stem 5. The arm 7carries at its mid-portion a roller 22 that is in contact with the cam6, which is located over the arm 7.

The portion of the adjusting screw 15 protruding from the nut member 12has a stopper portion 23 having a diameter larger than the innerdiameter of the nut member 12. As shown in FIG. 2, the stopper portion23 has a tapered surface 24 on its outer periphery which is adapted toabut a tapered surface 25 formed at the open end of the nut member 12when the adjusting screw 15 is pushed into the nut member 12, therebyrestricting the movement of the adjusting screw 15.

The operation of the lash adjuster 1 is now described.

When the cam 6 is rotated by the engine and the cam lobe 6 a of the cam6 presses down the arm 7, the valve 4 separates from the valve seat 10,thus opening the intake port 3. At this time, force acts on theadjusting screw 15 that tends to push in the adjusting screw 15. But dueto the frictional resistance between the external thread 14 of theadjusting screw 15 and the internal thread 13 of the nut member 12, theadjusting screw 15 is prevented from rotating, so that the adjustingscrew 15 is axially fixed in position.

When the cam 6 further rotates and the cam lobe 6 a moves past theroller 22, the valve stem 5 rises under the biasing force of the valvespring 9, until the valve 4 is seated on the valve seat 10 and theintake port 3 is closed.

In a strict sense, when the cam lobe 6 a of the cam 6 presses down thearm 7, slight slip occurs between the pressure flank 18 of the externalthread 14 and the pressure flank 18 of the internal thread 13. But afterthe cam lobe 6 a has moved past the roller 22 and until the cam lobe 6 aagain contacts the roller 22, since a load tending to push in theadjusting screw 15 is removed, the adjusting screw 15 returns to theoriginal position under the biasing force of the return spring 17.

When the distance between the cam 6 and the arm 7 increases due todifferences in thermal expansion between component parts of the valvegear such as the cylinder head 2, valve stem 5 and arm 7 while theengine is running, the adjusting screw 15 moves in the protrudingdirection while rotating under the biasing force of the return spring17. Thus, a gap never forms between the base circle 6 b of the cam 6 andthe roller 22.

Conversely, when the contact surfaces of the valve 4 and the valve seat10 become worn, even while the base circle 6 b of the cam 6 is incontact with the roller 22, the biasing force of the valve spring 9continuously acts on the adjusting screw 15 as a load tending to push inthe adjusting screw 15. Thus, due to slight slip that occurs between theexternal thread 14 and the internal thread 13 every time the cam lobe 6a contacts the roller 22, the adjusting screw 15 is gradually pushedinto the nut member, and the valve stem 5 gradually moves up, thuspreventing formation of a gap between the contact surfaces of the valve4 and the valve seat 10.

As discussed above, in an ordinary situation, only slight slip occursbetween the external thread 14 of the adjusting screw 15 and theinternal thread 13 of the nut member 12. But if abnormal loads orvibrations act on the adjusting screw 15 due to over-speed of theengine, if the frictional resistance between the pressure flanks 18decreases due to wear of the external thread 14 and the internal thread13, or if the adjusting screw 15 is suddenly and rapidly pushed in dueto sudden thermal expansion of the valve gear, unnecessarily large slipmay occur between the external thread 14 of the adjusting screw 15 andthe internal thread 13 of the nut member 12.

As a result, the adjusting screw 15 is markedly pushed into the nutmember 12. But as shown in FIG. 2, when the adjusting screw 15 is pushedin a certain distance, the stopper portion 23 abuts the open end of thenut member 12, thus preventing any further movement of the adjustingscrew 15, and thus preventing breakage of the return spring 17 due toover-compression or separation of the arm 7.

When abnormal loads or vibrations or sudden thermal expansion of thevalve gear disappears, the adjusting screw 15 is moved in the protrudingdirection under the biasing force of the return spring 17, and the lashadjuster returns to normal.

With this lash adjuster 1, since the adjusting screw 15 has a stopperportion 23 at its portion protruding from the nut member 12, it is notnecessary to provide a tubular extension for restricting the movement ofthe adjusting screw 15 by abutting the bottom member 16 at the end ofthe adjusting screw 15 located in the nut member 12. Thus, the externalthread 14 on the outer periphery of the adjusting screw 15 can be formedeasily by rolling.

With this lash adjuster 1, when the adjusting screw 15 is pushed intothe nut member 12, the tapered surface 24 on the outer periphery of thestopper portion 23 abuts the tapered surface 25 formed at the open endof the nut member 12, thereby preventing inclination of the adjustingscrew 15 by the contact of the tapered surfaces 24 and 25. This helpsthe adjusting screw 15 to protrude smoothly thereafter.

As shown in FIG. 3, the stopper portion 23 may have on its outerperiphery a flat surface 26 perpendicular to the axial direction andconfigured to abut a flat surface 27 formed on the open end of the nutmember 12 so as to be perpendicular to the axial direction when theadjusting screw 15 is pushed into the nut member 12. With thisarrangement too, it is possible to prevent inclination of the adjustingscrew 15 when the adjusting screw 15 is pushed into the nut member 12 bythe contact of the flat surfaces 26 and 27, thus helping the adjustingscrew 15 to protrude smoothly thereafter.

In the above embodiment, the stopper portion 23, which has a largerdiameter than the inner diameter of the nut member 12, is integrallyprovided at the portion of the adjusting screw 15 protruding from thenut member 12. But instead, the stopper portion 23 may be a separatemember from the adjusting screw 15. For example, such a separate stoppermember 23 may be a snap ring 28 fitted around the portion of theadjusting screw 15 protruding from nut member 12, as shown in FIG. 4.

In the above embodiment, the portion of the adjusting screw 15 havingthe external thread 14 on the outer periphery is integral with itsportion protruding from the nut member 12. But as shown in FIG. 5, theadjusting screw 15 may comprise a body 15 b having the external thread14 on the outer periphery, and a head 15 a separate from the body 15 band protruding from the nut member 12. In this case, the head 15 a maybe slidably inserted in the nut member 12, and an engaging member 29 maybe provided at the open end of the nut member 12 to prevent separationof the head 15 a from the nut member 12 by engaging the head 15 a.

FIG. 6 shows a valve gear including the lash adjuster 31 according tothe second embodiment of the present invention. As with the firstembodiment, this valve gear includes a valve 34 provided at an intakeport 33 of a cylinder head 32, and a valve stem 35 connected to thevalve 34. The valve stem 35 extends upwardly from the valve 34, and hasa spring retainer 36 fixed to its upper portion. A spring retainer 36biases the spring retainer 36 upwardly, thereby seating the valve 34against a valve seat 38.

The lash adjuster 31 comprises a lifter body 40 vertically slidablyinserted in a guide hole 39 formed in the cylinder head 32, a nut member41 vertically movable together with the lifter body 40, an adjustingscrew 44 having an external thread 43 on the outer periphery thereofwhich is in threaded engagement with an internal thread 42 formed on theinner periphery of the nut member 41, and a return spring 45 biasing theadjusting screw 44 in the direction to protrude downwardly from the nutmember 41.

The lifter body 40 comprises a tubular portion 46 and an end wall 47closing the top end of the tubular portion 46. A hard shim 48 is fixedto the top surface of the end wall 47. A cam 49 is in contact with theshim 48. The nut member 41 is integrally formed at the central portionof the end wall 47, and has its top end closed by the shim 48.

The external thread 43 and the internal thread 42 each have a pressureflank for receiving an axial load tending to push the adjusting screw 44into the nut member 41, and a clearance flank, and have aserration-shaped section, with the pressure flank having a larger flankangle than the clearance flank.

The return spring 45 is a compression spring having its top endsupported by the shim 48 and applying at its bottom end an axial forceto the adjusting screw 44 through a spring seat 50 that tends to pushthe adjusting screw 44 downwardly out of the nut member 41. The end ofthe adjusting screw 44 protruding from the nut member 41 is pressedagainst the top end of the valve stem 35. The end of the adjusting screw44 located in the nut member 41 has a tubular shape in which the springseat 50 is received.

The adjusting screw 44 has at its portion protruding from the nut member41 a stopper portion 51 having a larger diameter than the inner diameterof the nut member 41. When the adjusting screw 44 is pushed into the nutmember 41, the stopper portion 51 restricts the movement of theadjusting screw 44 by abutting the open end of the nut member 41. Thestopper portion 51 can be formed by rolling simultaneously with theexternal thread 43.

As with the first embodiment, with this lash adjuster 31, in an ordinarysituation, only slight slip occurs between the external thread 43 of theadjusting screw 44 and the internal thread 42 of the nut member 41. Butif abnormal loads or vibrations act on the adjusting screw 44 due toover-speed of the engine, or if the frictional resistance between thepressure flanks decreases due to wear of the external thread 43 and theinternal thread 42, unnecessarily large slip may occur between theexternal thread 43 of the adjusting screw 44 and the internal thread 42of the nut member 41.

As a result, the adjusting screw 44 is markedly pushed into the nutmember 41. But when the adjusting screw 44 is pushed in a certaindistance, the stopper portion 51 abuts the open end of the nut member41, thus preventing any further movement of the adjusting screw 44, andthus preventing breakage of the return spring 45 due toover-compression.

With this lash adjuster 31, since the stopper portion 51 for restrictingthe movement of the adjusting screw 44 is provided at the portion of theadjusting screw 44 protruding from the nut member 41, it is notnecessary to provide a tubular extension for restricting the movement ofthe adjusting screw 44 by abutting the shim 48 at the end of theadjusting screw 44 located in the nut member 41. Thus, the externalthread 43 on the outer periphery of the adjusting screw 44 can be formedeasily by rolling.

In this embodiment, the nut member 41 and the lifter body 40 areintegrally formed. But the nut member 41 and the lifter body 40 may beseparate members with the nut member 41 fixed to the lifter body 40. Ineither case, it is important that the nut member 41 move verticallytogether with the lifter body 40 when the lifter body 40 is verticallymoved.

FIG. 7 shows a valve gear including the lash adjuster 61 according tothe third embodiment of the present invention. This valve gear includesa valve 64 provided at an intake port 63 of a cylinder head 62 of anengine, a valve stem 65 connected to the valve 64, and an arm 67pivotally supported about a pivot shaft 66. The valve stem 65 extendsupwardly from the valve 64, and has a spring retainer 68 fixed to itsupper portion. A valve spring 69 biases the spring retainer 68 upwardly,thereby seating the valve 64 on a valve seat 70.

The arm 67 has its central portion pivotally supported by the pivotshaft 66. The arm 67 carries at one end thereof a roller 72 kept incontact with a cam 71 so that the arm 67 pivots as the cam 71 rotates.The lash adjuster 61 is mounted to the other end of the arm 67.

The lash adjuster 61 comprises a nut member 73, an adjusting screw 74and a return spring 75. The nut member 73 is inserted in a hole 76extending vertically through the arm 67. The adjusting screw 74 has anexternal thread 78 on its outer periphery that is in threaded engagementwith an internal thread 77 formed on the inner periphery of the nutmember 73.

The nut member 73 has a top end protruding from the top surface of thearm, and a tubular cap 79 having a closed top end is fitted on and fixedto the protruding top end portion of the nut member 73. The cap 79engages the top edge of the hole 76, thereby preventing the nut member73 from separating downwardly from the hole 76. The nut member 73 has aflange 80 at its bottom end which is in abutment with the bottom surfaceof the arm 67, and configured to receive upward force applied to the nutmember 73.

The external thread 78 and the internal thread 77 each have a pressureflank for receiving an axial load tending to push the adjusting screw 74into the nut member 73, and a clearance flank, and have aserration-shaped section, with the pressure flank having a larger flankangle than the clearance flank.

The return spring 75 is a compression spring having its top endsupported by the cap 79 and applying at its bottom end an axial force tothe adjusting screw 74 through a spring seat 81 that tends to push theadjusting screw 74 downwardly out of the nut member 73. The end of theadjusting screw 74 protruding from the nut member 73 is pressed againstthe top end of the valve stem 65.

The adjusting screw 74 has at its portion protruding from the nut member73 a stopper portion 82 having a larger diameter than the inner diameterof the nut member 73. When the adjusting screw 74 is pushed into the nutmember 73, the stopper portion 82 restricts the movement of theadjusting screw 74 by abutting the open end of the nut member 73. Thestopper portion 82 can be formed by rolling simultaneously with theexternal thread 78.

As with the first embodiment, with this lash adjuster 61, in an ordinarysituation, only slight slip occurs between the external thread 78 of theadjusting screw 74 and the internal thread 77 of the nut member 73. Butif abnormal loads or vibrations act on the adjusting screw 74 due toover-speed of the engine, or if the frictional resistance between thepressure flanks decreases due to wear of the external thread 78 and theinternal thread 77, unnecessarily large slip may occur between theexternal thread 78 of the adjusting screw 74 and the internal thread 77of the nut member 73.

As a result, the adjusting screw 74 is markedly pushed into the nutmember 73. But when the adjusting screw 74 is pushed in a certaindistance, the stopper portion 82 abuts the open end of the nut member73, thus preventing any further movement of the adjusting screw 74, andthus preventing breakage of the return spring 75 due toover-compression.

With this lash adjuster 61, since the stopper portion 82 for restrictingthe movement of the adjusting screw 74 is provided at the portion of theadjusting screw 74 protruding from the nut member 73, it is notnecessary to provide a tubular extension for restricting the movement ofthe adjusting screw 74 by abutting the cap 79 at the end of theadjusting screw 74 located in the nut member 73. Thus, the externalthread 78 on the outer periphery of the adjusting screw 74 can be formedeasily by rolling.

In the first embodiment, a compression spring is used as the returnspring 17 for biasing the adjusting screw 15 in the direction toprotrude upwardly from the nut member 12, a torsion coil spring may usedinstead as shown in FIG. 8.

In FIG. 8, the return spring 17 has its bottom end engaged in anengaging hole 83 formed in the bottom member 16, and its top end engagedin an engaging hole 84 formed in the adjusting screw 15. Thus thisreturn spring applies torque to the adjusting screw 15 that tends topush the adjusting screw out of the nut member 12 by its torsionaldeformation.

In the second embodiment too, as shown in FIG. 9, a torsion coil springmay be used as the return spring 45 for biasing the adjusting screw 44in the direction to protrude downwardly from the nut member 41.

In FIG. 9, the return spring 45 has its top end engaged in an engaginghole 85 formed in the shim 48 and its bottom end engaged in an engaginghole 86 formed in the adjusting screw 44. Thus this return springapplies torque to the adjusting screw 44 that tends to push theadjusting screw out of the nut member 41 by its torsional deformation.The external thread 43 on the outer periphery of the adjusting screw 44and the internal thread 42 on the nut member 41 are both verticallysymmetrical trapezoidal thread.

In the third embodiment too, as shown in FIG. 10, a torsion coil springmay be used as the return spring 75 for biasing the adjusting screw 74in the direction to protrude downwardly from the nut member 73.

In FIG. 10, the return spring 75 has its top end engaged in an engaginghole 87 formed in the cap 79 and its bottom end engaged in an engaginghole 88 formed in the adjusting screw 74. Thus this return springapplies torque to the adjusting screw 74 that tends to push theadjusting screw out of the nut member 73 by its torsional deformation.The external thread 78 on the outer periphery of the adjusting screw 74and the internal thread 77 on the nut member 73 are both verticallysymmetrical triangular thread.

FIG. 11 shows a valve gear including the lash adjuster 91 according tothe fourth embodiment of this invention. Elements corresponding to thoseof the first embodiment are denoted by identical numerals and theirdescription is omitted.

The adjusting screw 15 comprises a pivot member 15A axially slidablyinserted in the nut member 12, an externally threaded member 15Bsupporting the end of the pivot member 15A located in the nut member 12and having an external thread on its outer periphery, and a springwasher 15C disposed between the pivot member 15A and the externallythreaded member 15B. The spring washer 15C may be a disk spring washeror a wavy washer.

An annular anti-separation member 92 is fitted on the open end of thenut member 12 through which the pivot member 15A extends. The pivotmember 15A is formed with an annular groove 93 at its portion extendingthrough the open end of the nut member 12. The anti-separation member 92is engaged in the annular groove 93, thereby preventing separation ofthe pivot member 15A from the nut member 12.

At its portion protruding from the nut member 12, the pivot member 15Ahas a stopper portion 23 having a diameter larger than the innerdiameter of the nut member 12. The stopper portion 23 has a flat surface94 formed on its outer periphery to extend perpendicular to the axialdirection. When the pivot member 15A is pushed into the nut member 12,the flat surface 94 is configured to abut a flat surface 95 formed onthe anti-separation member 92 provided at the open end of the nut member12 to extend perpendicular to the axial direction, thereby restrictingthe movement of the pivot member 15A.

The pivot member 15A is formed with a square hole 96 in the center ofits end inserted in the nut member 12. The externally threaded member15B is formed with a square shaft 97 engaged in the square hole 96.Thus, when the pivot member 15A is rotated, the pivot member 15A and theexternally threaded member 15B rotate together due to the engagement ofthe square shaft 97 in the square hole 96.

The return spring 17 is a torsion coil spring having its bottom endengaged in an engaging hole 98 formed in the bottom member 16, and itstop end engaged in an engaging hole 99 formed in the externally threadedmember 15B. The return spring 17 thus applied torque to the externallythreaded member 15B that tends to push the pivot member 15A out of thenut member 12 due to its torsional deformation.

With this lash adjuster 91, when the adjusting screw 15 is pushed intothe nut member 12, the stopper portion 23 abuts the anti-separationmember 92 at the open end of the nut member 12, thus restricting themovement of the adjusting screw 15. This prevents breakage of the returnspring 17 due to over-compression.

With this lash adjuster 91, since the stopper portion 23 for restrictingmovement of the adjusting screw 15 is provide at the portion of thepivot member 15A protruding from the nut member 12, it is not necessaryto provide a tubular extension at the end of the externally threadedmember 15B located in the nut member 12 for restricting the movement ofthe adjusting screw 15 by abutting the bottom member 16. Thus, theexternal thread 14 on the externally threaded member 15B can be formedeasily by rolling.

With this lash adjuster 91, when the engine is stopped in ahigh-temperature state, and differences in shrinkage appear betweencomponent parts of the valve gear when the engine cools downsubsequently, the spring washer 15C between the externally threadedmember 15B and the pivot member 15A is compressed, thereby absorbingsuch differences in shrinkage. Thus, when the engine is restarted, nogap forms between the valve 4 and the valve seat 10 due to differencesin shrinkage. This prevents compression leakage.

With this lash adjuster 91, since the pivot member 15A and theexternally threaded member 15B are adapted to rotate together due to theengagement of the square shaft 97 in the square hole 96, it is possibleto rotate the externally threaded member 15B by rotating the pivotmember 15A. Thus, compared to an arrangement having no square shaft 97and the square hole 96, the externally threaded member 15B can be easilymounted in the nut member 12.

In this embodiment, the elastic member disposed between the pivot member15A and the externally threaded member 15B is the spring washer 15C. Butthe spring washer 15C may be replaced by a different elastic member(such as a compression coil spring).

If a torsion coil spring is used as the return spring 17, it may be atubularly wound member as shown in FIG. 11, or may be a conically woundmember as shown in FIG. 12.

Also, the return spring 17 may be a torsion spring other than a torsioncoil spring (such as a spiral spring or a volute spring).

The return spring 17 in FIG. 13 is a spiral spring comprising a spirallywound thin sheet. This return spring 17 has its radially outer endrotationally fixed to the bottom member 16 of the nut member 12 and itsradially inner end engaged in a slit formed in a protrusion 100 formedon the end of the externally threaded member 15B located in the nutmember 12. The return spring 17 thus applies torque to the externallythreaded member 15B that tends to push the pivot member 15A out of thenut member 12 due to its torsional deformation. The external thread 14on the outer periphery of the externally threaded member 15B and theinternal thread 13 on the inner periphery of the nut member 12 arevertically symmetrical triangular threads.

The return spring 17 in FIGS. 14 and 15 is a volute spring comprising ahelically wound thin sheet. This return spring 17 has its radially outerend rotationally fixed to the bottom member 16 of the nut member 12 andits radially inner end engaged in a slit formed in a protrusion 101formed on the end of the externally threaded member 15B located in thenut member 12. The return spring 17 thus applies torque to theexternally threaded member 15B that tends to push the pivot member 15Aout of the nut member 12 due to its torsional deformation. The externalthread 14 on the outer periphery of the externally threaded member 15Band the internal thread 13 on the inner periphery of the nut member 12are vertically symmetrical trapezoidal threads.

1. A lash adjuster comprising a nut member (12) having an internalthread (13) on its inner periphery, an adjuster screw (15) having anexternal thread (14) on its outer periphery which is in threadedengagement with the internal thread (13), and a return spring (17)biasing the adjusting screw (15) in a direction to protrude from the nutmember (12), wherein an axial load that tends to push the adjustingscrew (15) into the nut member (12) is received by pressure flanks (18)of the external thread (14) and the internal thread (13), characterizedin that the adjusting screw (15) has a stopper portion (23) at itsportion protruding from the nut member (12), said stopper portion (23)having a diameter larger than the inner diameter of the nut member (12),wherein the nut member (12) has an open end configured to support thestopper portion (23), thereby restricting movement of the adjustingscrew (15) when the adjusting screw (15) is pushed into the nut member(12).
 2. The lash adjuster of claim 1 wherein the stopper portion (23)has a tapered surface (24) on its outer periphery which is configured toabut a tapered surface (25) formed on the open end of the nut member(12).
 3. The lash adjuster of claim 1 wherein the stopper portion (23)has a flat surface (26) on its outer periphery which extendsperpendicular to an axial direction and configured to abut a flatsurface (27) formed on the open end of the nut member (12) so as toextend perpendicular to the axial direction.
 4. The lash adjuster ofclaim 1 wherein the stopper portion (23) comprises a snap ring (28)fitted on the outer periphery of the portion of the adjusting screw (15)protruding from the nut member (12).
 5. The lash adjuster of claim 1wherein the return spring (17) is a compression spring that applies anaxial force to the adjusting screw (15) that tends to push the adjustingscrew (15) out of the nut member (12), and wherein the external thread(14) and the internal thread (13) have a serration-shaped section withtheir pressure flanks (18) having a larger flank angle than theirclearance flanks (19).
 6. The lash adjuster of claim 1 wherein thereturn spring (17) is a torsion spring that applies torque to theadjusting screw (15) that tends to push the adjusting screw (15) out ofthe nut member (12), and wherein the external thread (14) and theinternal thread (13) are serration-shaped threads, trapezoidal threadsor triangular threads.
 7. The lash adjuster of claim 6 wherein thetorsion spring is one of a torsion coil spring, a spiral spring and avolute spring.
 8. The lash adjuster of claim 1 wherein the nut member(12) is inserted in a receiving hole (11) formed in a top surface of acylinder head (2), and wherein the adjusting screw (15) pivotallysupports an arm (7) of a valve gear at its end protruding from the nutmember (12).
 9. The lash adjuster of claim 1 wherein the nut member (41)is fixed to a lifter body (40) vertically slidably inserted in a guidehole (39) formed in a cylinder head (32), and wherein the adjustingscrew (44) presses a valve stem (35) of a valve gear at its endprotruding from the nut member (41).
 10. The lash adjuster of claim 1wherein the nut member (73) is inserted in a receiving hole (76) formedin a bottom surface of an arm (67) that pivots as a cam (71) rotates,and wherein the adjusting screw (74) presses a valve stem (65) of avalve gear at its end protruding from the nut member (73).
 11. The lashadjuster of claim 1 wherein the adjusting screw (15) comprises a pivotmember (15A) axially slidably inserted in the nut member (12), anexternally threaded member (15B) supporting an end of the pivot member(15A) located in the nut member (12) and having said external thread(14) on its outer periphery, and an elastic member (15C) disposedbetween the pivot member (15A) and the externally threaded member (15B).12. The lash adjuster of claim 11 wherein the pivot member (15A) has asquare hole (96) formed in its end located in the nut member (12), andwherein the externally threaded member (15B) has a square shaft (97)engaged in said square hole (96), whereby the pivot member (15A) and theexternally threaded member (15B) are rotatable together due to theengagement of the square shaft (97) in the square hole (96).